The present invention is an article which can automatically make a screen analysis of a granular material. It is only necessary for an operator to introduce a sample and start the apparatus. In its preferred form, no manual weighing or calculations are necessary.
Particle-size analysis is a standard industrial test which is used on materials as varied as cereal grains, concrete aggregate, and wood chips, to draw just a few examples. Many items of commerce are sold and purchased on the basis of some predetermined size classification. Size classification data is also frequently used as input to control various manufacturing processes. This classification is usually referred to as a screen analysis, a term chosen from the laboratory method of determining the particle-size distribution. The most common method of running a screen analysis is to assemble a stack of superposed screens of graded sizes. The uppermost screen will be the coarsest and the lowermost the finest in terms of opening size. The sample to be tested is placed upon the top screen, whereupon the nest of screens is shaken for some predetermined period. After the shaking period, the material remaining on each screen and on the bottom pan is individually weighed and these weights are used to calculate a screen analysis.
A number of industrial screening processes are also based upon the concept of stacked screens of various opening sizes. In this case, the screens are usually inclined so that the fraction contained on any given screen ultimately falls from the end of the screen into a predetermined bin or onto a conveyor. Another common industrial screen is the inclined rotary drum. The surface of the drum is made of screens which may also be of sizes graded from relatively fine at the entrance end to relatively coarse at the discharge end. Once again, the various fractions are permitted to fall into separate bins or are carried away on different conveyor systems.
The development of rotary screening devices appears to have reached a mature status many years ago. Two relatively recent U.S. patents might be cited as exemplary of recent work. Satake, U.S. Pat. No. 3,750,884, shows a rice sorter which includes two polygonal drum type screens having slotted openings in a circumferential pattern. Gauld, U.S. Pat. No. 4,043,901, shows a wood chip cleaner for use on materials such as forest residual chips. This device is based on a rotary drum that separates the chips from trash and then further classifies the chips into acceptable and oversize fractions. Gauld includes a useful state-of-the-art description of rotary screens for wood chips.
In the past twenty years, the change that has taken place in the automation of laboratory testing procedures may properly be described as revolutionary. Surprisingly, this revolution has not affected screen analysis determination. This is still carried on essentially manually in the same fashion as it has been from its inception. A few new types of apparatus have been developed, but these have not been particularly useful in taking away the many manual weighing operations. U.S. Pat. No. 4,141,451 to Lapointe is such an example. This is a device for classifying wood chips by thickness. It is based upon the use of a plurality of adjustable shingled bars that form the periphery of a rotating drum. A sample is inserted into the drum which is then rotated for a given period of time with the bars set at a given spacing. The drum is then stopped, the bars readjusted to the next spacing, and rotation resumed for the prescribed time. Chips of a predetermined thickness class fall between the bars onto an oscillating inclined screen where they are further classified depending primarily upon their length or width. At the end of the test run, the individual screen fractions are collected and manually weighed.
Since the determination of screen analysis tends to be a labor intensive operation, the need has existed for a device to make such analyses on a fully automated basis. The present invention is a particle-size classifier which fills the above need. All that is necessary is to place a sample in the apparatus and start the screening operation. The end result is a printout or digital display that can give either a percentage or weight analysis of the individual screen fractions.